Electroless copper plating holds great promise as a method to form a copper film for ULSI fine wiring, and as a replacement for the sputtering and electrolytic copper plating methods currently in use.
Conventionally, when a semiconductor wafer or other such mirror surface was electroless plated with copper, it was difficult to obtain a good adhesion of the deposited plating film. Also, the plating reactivity was low and it was difficult to plate uniformly over the entire substrate. Examples of problems currently encountered in electroless copper plating include a low adhesive strength and poor plating uniformity when a copper film is formed over a barrier metal layer such as tantalum nitride.
Formalin is typically used as a reducing agent for an electroless copper plating solution, but because formalin is harmful to humans and the environment, glyoxylic acid, which shows a similar reaction mechanism, has been studied in recent years as a possible alternative. An electroless copper plating solution in which glyoxylic acid is used as a reducing agent was disclosed in Japanese Patent Publication No. 2002-249879, the object of which was to provide an electroless copper plating solution that could be used stably over an extended period and, in the solution, glyoxylic acid was used as a reducing agent, potassium hydroxide was used as a pH regulator, and methanol, a primary amine, or the like was used as a Cannizzaro's reaction inhibitor.